From tundra to tree-birch; late-glacial and early Holocene environment and vegetation oscillations at the ecotonal positioned Bjerkreim, Dalane, SW Norway

Abstract

Basin sediments from Bjerkreim, SW Norway, have been analyzed regarding pollen, plant macrofossils, chironomids, palaeomagnetism, elements, volcanic ash, and loss-on-ignition. The chronology is based on the Vedde Ash and Saksunarvatn ash layers and radiocarbon dates of plant and insect macrofossils. The records spanning the period from local deglaciation around 16 ka BP to the early Holocene ca 10.2 ka BP, have given new detailed information of the late-glacial environments. In the cold pre-Bølling (prior to 14.6 ka BP), open pioneer vegetation on unstable mineral soils was dominant. After the Bølling warming, climate became moister and reached the late-glacial thermal maximum (July mean of 12–13 °C), and dominant dwarf shrubs and snow beds established. Shallow soils, strong winds, and drought may have restricted vegetation development. Despite colder summers (July mean of 9.5–11 °C) in Allerød 13.9–12.8 ka BP, fertile humus-soils developed, and shrub vegetation and open birch-forests established. Snow beds regained dominance in Younger Dryas (July mean of 6.5–9 °C). The early Holocene warming initiated rapid establishment of early Betula/Populus-forests and the later Corylus forests around 10.7 ka BP. Three cooling events in the Bølling-Allerød interstadial (Older Dryas/GI-1d, G1-1c2, GI-1b/Gerzensee) temporarily increased open-ground vegetation showing the ecotonal position of Bjerkreim with low resistance to these changes. The Empetrum expansion was delayed (from 14.5 to 13.7 ka BP) towards the east along the coast of Rogaland. This could point to a precipitation/outwash gradient decreasing towards the east and developing the acid soils necessary for Empetrum to expand. For the first time, macrofossils show the late-glacial presence of tree-birch in Norway. Tree-birch were sparse in Bølling and formed patches of open-forests in Allerød. Several pollen taxa previously thought to reflect long-distance dispersal, such as Populus, Hippohaë, Jasione, and Sanguisorba ssp., most probably were present in Rogaland during Late-glacial. Doggerland seem to have been important for the late-glacial northwards migration of plants into South Norway.